The mid-infrared (2-16 μm) part of the electromagnetic spectrum offers significant opportunities for exciting scientific breakthroughs and technological advances because it is a key enabler of new advanced sensing technologies. However, the lack of bright sources of mid-infrared radiation has limited the exploitation of this part of the electromagnetic spectrum. The unfavourable laser dynamics of rare earth doped laser sources that operate in this part of the spectrum often make them inefficient, resulting in low output powers for practical use.
Virtually all mid-infrared laser transitions occur between energy levels that are located well above the ground state. FIG. 1A shows a schematic energy level diagram 10′ of a prior art single wavelength pumping system for generating mid-infrared laser radiation. In standard operation the ions are excited from the ground state 1 to the upper laser level 3 using a near infrared, or visible light pump source 4. An ion in the excited upper laser level 3 then undergoes a laser transition 5 emitting a laser photon and leaving the ion in a long lived post lasing excited state 2. After some delay the ion decays back to the ground state 6 releasing waste energy in the process. Once an ion returns to the ground state it can again be re-excited 8 back to the upper lasing state 3 by pump 4. The cyclic nature of this process is illustrated by dotted line 7 which links the excitation phase to the lasing transition and decay to ground state phase, and dotted line 8, which links this phase back to the excitation phase.
Whilst this single wavelength pumping approach can be used to directly excite the laser ions from the ground state 1, the long lifetime of ions in the long lived post lasing excited state 2 causes serious bottlenecks of ions building up in this state. For example, in rare earth doped fibre lasers, this long lived post lasing excited state 2 can have lifetimes of hundreds of microseconds or several milliseconds (or more). This build up or bottle neck of ions in the long lived post lasing excited state 2 results in termination of the lasing process due to failure to maintain the required population inversion between the upper state 3 and long lived post lasing excited state 2, and thus limits the available lasing power of such systems. The long lived post lasing state can seriously limit laser performance even if it is not the lower lasing state because ions that collect here are not returned promptly to the ground state to absorb the pump light leading to reduced excitation rate to the upper lasing state and poorer efficiency. Further, the direct excitation approach results in poor optical efficiency and significant waste heat energy as the ions return 6 to the ground state 1 from the long lived post lasing excited state 2.
FIG. 2A is a schematic energy level diagram 11 of an erbium doped ZBLAN fibre illustrating a pumping arrangement (P) using a 655 nm DCM dye laser to excite ions from the 4I15/2 ground state to the 4F9/2 state, or energy level (the terms state and energy level will be used interchangeably). A lasing transition (L) from the 4F9/2 level to the 4I9/2 level generates a 3.5 μm laser output. This system resulted in an 8 mW output with a slope efficiency of less than 3% at room temperature. Once the lasing transition has occurred the ions rapidly decay from the 4I9/2 levels and bottleneck in the 4I11/2 and 4I13/2 levels that have long lifetimes of ˜6.5 ms and ˜8.5 ms respectively. This prevents the ground state from being replenished which reduces pump radiation absorption and the repopulation of the upper lasing level, 4F9/2 resulting in very poor laser slope efficiency.
Several approaches have been attempted to address the significant problem of ions building up in long lived post lasing excited states of the medium, and thus improve the efficiency of laser systems operating in the mid infrared region. These have sought to deplete the population of ions in the long lived excited state by a promoting or inducing a beneficial up-conversion process in which an ion in the long lived state is promoted to a higher state such as by stimulation with a further pump or by co-doping the medium with other rare earth ions. Up-conversion has been shown to work for specific laser transitions with particular dopants but adversely affects the efficiency of other transitions because of reductions in the lifetime of upper lasing state. Co-dopants can also be used to reduce the bottlenecks in the system but do not in general improve the Stokes efficiency of the laser.
There is thus a need to develop pumping systems and methods to improve the efficiency of laser systems operating in the mid infrared region, or at least to provide users with a useful alternative to existing laser systems.